A radome is a structure which envelopes and protects a radar antenna from the environment and, desirably, causes very little interference with the signal. Missile radomes are ogival or bullet-shaped shells which shield the antenna from high air velocities, rain, etc. Radomes fabricated from fiber glass have proven satisfactory for missiles operating at low velocities. With high velocity missiles, however, where greater surface heating, larger loading forces, and more severe rain erosion are encountered, radomes manufactured from ceramic materials have been utilized. Ceramic radomes must have the capability of being shaped through grinding to a very specifically-defined prescription, this prescription being designed to insure that the resistance to the signal of the antenna is uniform in every direction.
For over 20 years Corning Glass Works, Corning, N. Y. has manufactured radomes for radar guided missiles from a glass-ceramic material marketed as Corning 9606. That product is highly crystalline with cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) constituting the predominant crystal phase with minor amounts of cristobalite (a polymorph of SiO.sub.2), rutile (TiO.sub.2), and a phase until recently believed to be magnesium dititanate (MgO.2TiO.sub.2) being present also. An approximate analysis of the material, expressed in weight percent on the oxide basis, is reported below:
______________________________________ SiO.sub.2 56.1 Al.sub.2 O.sub.3 19.7 MgO 14.9 As.sub.2 O.sub.3 0.4 TiO.sub.2 8.9 ______________________________________
To be useful as a radome, a material must comply with a complex matrix of mechanical, electrical, thermal, and forming properties, several of the most important of which are discussed in the following text.
The candidate glass-ceramic should exhibit a low loss tangent. The loss tangent defines the quantity of energy absorbed by a material from radiation passing therethrough. High loss tangents have the effect of reducing the range of the radar. Furthermore, not only is the magnitude of the loss tangent significant, but the level thereof should be reasonably stable over the range of temperatures to be encountered by the material. Corning 9606 demonstrates a loss tangent at 8.6.times.10.sup.9 Hz of 0.00030 at 25.degree. C.
The wall thickness of a radome is dictated by three factors: loss tangent, the dielectric constant of the material, and the particular wavelength of radiation being employed. Interference with the signal will be at a minimum if the thickness of the wall is a multiple of one-half a wavelength. The dielectric constant affects the velocity of the radiation and, hence, the wavelength thereof in the glass-ceramic. When the loss tangent is kept minimal, the thickness of the wall can be increased with decreasing values of dielectric constant. Also, as with loss tangent, the dielectric constant ought not to vary greatly as the temperature of the radome rises. Corning 9606 has a dielectric constant of 5.5 at room temperature (25.degree. C.) and 8.6.times.10.sup.9 Hz, and that value is substantially independent of temperature up to about 750.degree. C. As noted above, dielectric constants greater than 5.5 would require somewhat thinner wall thicknesses and such would result in greater internal heating of the apparatus within the radome.
The radome material must exhibit high mechanical strength to support attachment to the missile, to survive the vibration which occurs during launch and flight, and to aid in overcoming thermal stress, the highest stresses being thermal in origin. Resistance to thermal shock is directly related to the mechanical strength, the elastic properties, and the coefficient of thermal expansion of a material. In general, the lower the coefficient of thermal expansion of a material, the greater will be its resistance to thermal shock. Corning 9606 displays an average coefficient of thermal expansion (25.degree.-300.degree. C.) of about 57.times.10.sup.-7 /.degree. C.
Because the newer missiles fly at higher velocities and under more severe conditions, an improved radome material is demanded which will demonstrate greater thermal shock resistance than Corning 9606, while exhibiting mechanical, electrical, and forming properties similar to those of Corning 9606.
U.S. Pat. No. 2,920,971, the basic patent in the field of glass-ceramics, discloses the production of glass-ceramic articles as involving the controlled crystallization of precursor glass articles through a carefully-defined heat treatment thereof. Hence, the formation of a glass-ceramic body comprehends three general steps: first, a glass-forming batch of the proper composition is melted; second, the melt is simultaneously cooled to a temperature at least below the transformation range thereof and a glass body of a desired geometry shaped therefrom; and, third, the glass body is subjected to a predetermined heat treatment at temperatures above the transformation range to cause the glass to crystallize in situ. Frequently, this third step is divided into two parts. Thus, the glass body will initially be heated to a temperature at or somewhat above the transformation range and held thereat for a sufficient length of time to cause the development of nuclei in the glass. Subsequently, the nucleated glass body is heated to a higher temperature, often above the softening point thereof, and maintained thereat for a period of time sufficient to effect the growth of crystals on the nuclei. This two-step practice customarily results in a more homogeneously crystallized material wherein the crystals are more uniformly sized.
The microstructure, the general characteristics, and the method for making glass-ceramic articles are discussed in considerable detail in U.S. Pat. No. 2,920,971, and that patent is specifically referred to for a fuller understanding of those features.
A very significant advantage which radomes fabricated from glass-ceramics possess, when compared with those fashioned through slip casting or other forming method for ceramic materials, is a high degree of chemical and structural homogeneity. Thus, the precursor glass can be melted to a very fine homogeneity and then dropped into a spinning mold to form the basic ogival article. Furthermore, deformation during crystallization in situ is very slight in contrast to that taking place during sintering of a ceramic body. Accordingly, grinding the radome structure to the demanded prescription is more easily accomplished than with a sintered ceramic body.
To impart the required mechanical strength to Corning 9606 radomes, the bullet-shaped structures, after grinding to the required prescription, are subjected to what has been termed a fortification treatment. That treatment comprises subjecting the glass-ceramic to a sequential base-acid leaching process. Thus, the radome is initially contacted with (normally immersed into) an alkaline solution and thereafter, after removing the alkaline solution, it is contacted with (immersed into) an acid solution. That series of steps may be repeated several times in order to achieve the desired effect. As a matter of convenience and economics, a boiling aqueous NaOH solution has constituted the alkaline environment and an aqueous, room temperature H.sub.2 SO.sub.4 solution has provided the acid conditions. The base and acid were customarily removed via rinsing in tap water.
The improvement in strength is deemed to result via healing surface flaws in the body. This phenomenon is due to the cristobalite being leached out of the microstructure (cristobalite is several times more quickly dissolved in hot NaOH solution than is cordierite). The acid acts upon the little residual glass left in the glass-ceramic body. After fortification, Corning 9606 demonstrates an average modulus of rupture of about 35,000 psi. A somehwat porous surface layer is developed which protects the radome body from surface abuse encountered in use that would reduce its strength. Some care must be exercised in carrying out the fortification process, however. Thus, excessive treatment causes a reduction in strength. Although the reason for this reduction has not been rigorously studied, it has been postulated that overstretching may lead to the development of new flaws in the body surface or simply that too much material is removed therefrom. Empirical observation has determined that a porous surface layer having a depth of about 0.005"-0.015" appears to yield the most desirable strength properties.
In general, glass-ceramic articles containing cordierite as the predominant crystal phase, but with little or no cristobalite, will demonstrate mechanical strengths, as defined through modulus of rupture measurements of less than 20,000 psi, whereas those cordierite-containing articles with a minor, but significant, amount of cristobalite will evidence modulus of rupture measurements in excess of 30,000 psi after fortification. X-ray diffraction analysis and electron microscopy have indicated that Corning 9606 contains about 10% by volume cristobalite.